It is well known in the art to extrude visco-elastic materials, such as rubber, using a screw extruder and a flow channel head for communicating the rubber through a flow channel from the extruder to a die. At the discharge end of the extruder the visco-elastic material flows over a screw nose into a receiving end of the flow channel. One problem known in the art, for example, in the rubber industry, is porosity, or the formation of bubbles in the material at the discharge end of the extruder where the screw flights terminate. The bubbles are carried in the material through the flow channel and show up as blisters, or porosity in the resulting rubber component after leaving the die.
It is believed the bubbles are formed in the space at the discharge end of the screw flights, because the bubbles show up at positions corresponding to the positions of the screw flights. The bubbles are also believed to result from shrinkage, or the tendency of visco-elastic material to revert to the molecular state of the material prior to the extrusion process, otherwise known as “memory.” When the material is being extruded it is stretched, and when the stretching stops, or is decreased, the material will tend to shrink. This reduction in pressure on the material is believed to cause the volatiles in the material to expand and produce bubbles. In the present invention, an enlarged screw nose is provided for preventing this creation of porosity in the extrudate.
Examples of extruders having enlarged screw noses are U.S. Pat. No. 4,357,291, which shows a conventional mixing section 26 for an extruder of the type used for extruding ethylene polymers. U.S. Pat. No. 5,660,864 shows a bullet nose at the end of a screw section of an injection piston for injection molding of glass fiber reinforced polyester material. U.S. Pat. No. 3,846,057 is directed to a reciprocating screw extruder for injection molding of rubber which has a head on the forward end of the screw cooperating with a check ring preventing back flow of the rubber during the injection process. In none of these patents is there a description or showing of an extruder screw with an enlarged screw nose having a conical surface for preventing porosity in the extrudate.